1. Field of the Invention
The present invention relates to a semiconductor device including power semiconductor devices and control semiconductor devices for controlling the power semiconductor devices and a method of fabricating the semiconductor device. The invention also relates to a composite substrate for use in the semiconductor device and a method of fabricating the composite substrate.
2. Description of the Background Art
FIG. 13 is a cross-sectional view of a conventional semiconductor power module, and FIG. 14 is a schematic cross-sectional view of a composite substrate for use in the power module. The power module 100 includes a composite substrate (insulating substrate) 110 on a copper base plate 101, as shown in FIG. 13. The composite substrate 110 includes a ceramic substrate 111 and copper plates 112 and 113 joined respectively to both surfaces of the ceramic substrate 111, as shown in FIG. 14. The copper plate 112 is patterned to the configurations of electronic circuits to be formed thereon so that it is separated into a first region 112a and a second region 112b. power semiconductor devices (referred to as "power devices" hereinafter) 103 used under high voltage and high-current conditions are mounted in the first region 112a, as shown in FIG. 13, so that a high-voltage circuit is formed. Control semiconductor devices (referred to as "control devices" hereinafter) 104 used under low-voltage and low-current conditions and for controlling the power devices 103 are mounted in the second region 112b (See FIG. 14). so that a control circuit (low-voltage circuit) is formed.
Referring to FIG. 13, an outsert case 130 having first and second connecting terminals 131 and 132 is bonded at its end portion 133 to the copper base plate 101. This permits the outsert case 130 to accommodate the composite substrate 110, power devices 103 and control devices 104. A first connecting terminal portion 131a connected electrically to the first connecting terminals 131 is soldered to the first region 112a of the copper plate 112, so that the first connecting terminals 131 and the power devices 103 are connected electrically to each other. A second connecting terminal portion 132a connected electrically to the second connecting terminals 132 is soldered to the second region 112b, so that the second connecting terminals 132 and the control devices 104 are connected electrically to each other
The lower portion of the interior of the outsert case 130 is filled with silicon gel 141, and the upper portion thereof is filled with epoxy resin 142.
In the power module 100, when a control signal is applied to the control devices 104 through the second connecting terminals 132, a signal from the control devices 104 is applied to the power devices 103 to control the drive of the power devices 103. This provides for the drive control of a load circuit (not shown) connected electrically to the power devices 103 through the first connecting terminals 131.
Since both of the high-voltage circuit and the control circuit are disposed on the single ceramic substrate 111 in the conventional power module 100, there is formed a capacitance between the high-voltage circuit and control circuit through the ceramic substrate 111. When noises are produced in the high-voltage circuit due to external factors such as load fluctuation, the noises are led to the control circuit through the capacitance in the ceramic substrate 111 to cause the faulty operations of the control devices 104. There arises a problem that the power devices 103 also faultily operate, resulting in deterioration of the reliability of the power module 100.